The invention relates to a clamping circuit for a video signal, the clamping circuit comprising a differential amplifier the output of which is fed back to an amplifier input via a diode and is also connected to the output of the clamping circuit, an input of the amplifier being connected to a terminal which carries a reference voltage, a further input for receiving a video signal being connected to a capacitor which is connected to a capacitor charging and discharging circuit.
Such a clamping circuit is disclosed in the German Offenlegungsschrift No. 2,433,452, which describes that the feed-back is effected from the circuit output to the amplifier input for receiving the video signal and is connected to the isolating capacitor, the terminal which carries the reference voltage being connected directly to the second amplifier input. It is described that at the occurrence of line synchronizing pulses in the video signal and on the passage of the reference voltage, the amplifier supplies a voltage which renders the diode conductive, in response to which the capacitor, via which the video signal is applied, is charged. The capacitor is thus charged to a voltage at which the level of the line synchronizing pulses in the output signal is fixed or, in other words, clamped onto the reference voltage.
It appears that the isolating capacitor, which is arranged between the input and the output of the clamping circuit, is charged via the diode during the line synchronizing pulses or, in other words: during clamping of the line synchronizing pulse level in the output signal onto the reference voltage the output is capacitively coupled to the input of the clamping circuit to which the video signal is applied.